(1) Field of the Invention
The present invention relates to a semiconductor device, and more particularly to improvement of a semiconductor device having a field plate electrode.
(2) Description of the Related Art
Semiconductor devices that use a field plate electrode to relax electric field concentration on a gate electrode have recently been applied to practical use (see, for example, Japanese Patent Application Laid-Open No. 2000-315804).
The structure of an example of such conventional semiconductor devices is shown in FIG. 1. In the figure, formed on a semiconductor substrate 1 is an operation layer 2, on which a source electrode 3 and a drain electrode 4 in ohmic contact therewith are disposed. A recess 5 is formed in the semiconductor layer between the source electrode 3 and the drain electrode 4. Formed in the recess 5 is a gate electrode 6 with a Schottky junction, and formed on a surface protection film 7 made of an insulating material is a field plate electrode 8.
Forming the gate electrode 6 in the recess 5 is for the purpose of keeping the Schottky interface of the gate electrode 6 away from the surface of the operation layer 2 to avoid the effect of the surface level so that the semiconductor device operates stably. The field plate electrode 8 is provided for relaxing electric field concentration on the gate electrode 6, and plays an important role in increasing the output of the semiconductor device.
To increase the effect of relaxing the electric field concentration, it is desirable that the field plate electrode 8 be disposed near the Schottky interface between the gate electrode 6 and the operation layer 2, and be lengthened in the channel direction.
However, in a conventional semiconductor device as shown in FIG. 1, the gate electrode 6 is formed in the recess 5 and the field plate electrode 8 is formed on the surface of the operation layer 2. As a result, the field plate electrode 8 is spatially far away from the gate electrode 6. The electric field concentration on the gate electrode 6 is therefore not sufficiently relaxed. Further, lengthening the field plate electrode 8 in the channel direction (C direction) increases the feedback capacitance between the gate and the drain. This makes it difficult to achieve stable operation of a semiconductor device, which may lead to causing oscillation.
As described above, a semiconductor device having a conventional field plate electrode as shown in FIG. 1 has a problem that a sufficient effect on relaxing the electric field concentration on the gate electrode cannot be achieved since the field plate electrode is spatially far away from the gate electrode. This causes damage to the stable operations of the semiconductor device.
A semiconductor device having a field plate electrode that is integrated with a gate electrode and projects toward a drain electrode by a predetermined distance on a surface protection film is known (see Japanese Patent Application Laid-Open No. 2001-144106). Such a semiconductor device, however, has a problem that a projecting portion needs to be lengthened and thinned in order to increase the effect of relaxing the electric field concentration.